The long-term objective of this project is to gain a better understanding of mammalian muscle cell differentiation, in vivo, using mice carrying a targeted mutation at the gene encoding the myogenic regulatory protein myogenin. Myogenin is a muscle-specific transcription factor belonging to the helix-loop-helix (bHLH) class of proteins and is believed to play a crucial role in the differentiation of muscle. tissue in mammalian embryos. The role of myogenin in muscle cell differentiation will be tested directly by making use of a mouse model with a myogenin-null genotype. Using homologous recombination techniques, mice have recently been obtained that are heterozygous for the targeted mutation. The myogenin-null mice will be the starting point for a series of experiments to delineate myogenin function in vivo. The specific aims are: (l) To characterize the phenotypes associated with myogenin-null mice, by assessing the biochemical, histological and physiological state of muscle and associated tissues in embryos, fetuses, and adults; (2) To determine the role of myogenin in regulating the expression of muscle-specific genes, including other myogenic factors and genes encoding muscle-specific structural proteins and enzymes; (3) To determine if autoregulation is required for myogenin expression, making use of pre-existing transgenic mice harboring myogenin gene control regions fused to reporter genes; (4) To determine if mice that are doubly homozygous for null mutations at the myogenin locus and other myogenic factor loci exhibit stronger phenotypes than any of the single homozygous mutations; (5) To determine if other myogenic factors or modified forms of myogenin, expressed in the same temporal and spatial fashion, can rescue the myogenin-null phenotypes; and (6) To create myogenin-null fibroblast and myoblast lines to study the potential of these cells to differentiate muscle.